The present invention relates generally to high pressure transducers and more specifically to an optical pressure sensor which produces an optical response that varies with its ambient environment, and measures pressures which are characterized by sudden extreme changes in pressure, such as a nuclear detonation.
The Minuteman intercontinental ballistic missiles in the United States are housed underground in concrete silos so that they may withstand the nuclear detonation of sea launched ballistic missiles and near misses of Soviet intercontinental ballistic missiles. These hardened silos allow the United States to withstand a first strike nuclear confrontation initiated by foreign powers, and strike back.
The sufficiency of the hardened silos is tested by a detonation of munitions in proximity with silo models to produce a shock wave that simulates a nuclear detonation. These tests need to be monitored by pressure sensors, but most commercially available pressure sensors which have an electrical output, do not have the adequate pressure range of electrical bandwidth to measure the extreme variations in pressure characterized by a simulated nuclear detonation during silo testing.
Most high pressure sensors are based on electrically sensing the elastic response of a body to an external pressure. For pressures well below the elastic limit of typical materials (nominally 100,000 psi), a macroscopic change is usually observed such as the flexing of a diaphragm. An electrical transducer such as a strain gauge is used to sense the deflection of the sensor. In piezoelectric sensors the elastic material and electric transducer are combined in one material. For higher pressures, where the microscopic structure of the material is strongly perturbed, it is often possible and preferable to directly measure the change in some material property (such as resistance).
For a sensor geometry which imposes tension on the elastic sense element, measurement is limited to pressures below its yield point; that is those tensions which do not cause permanent distortion. A material can usually withstand greater compressional stress than tensile stress if the material is free of defects and the pressure is applied uniformly from all directions. Sensors based on compression must be used above the yield point of the material.
Available sensors typically convert the strain in the elastic material to an electrical signal by means of an electrical transducer. All of the methods used to sense linear displacement (such as metallic or semiconductor strain gauges, changes in inductance or capacitance, acoustic frequency, Hall effect, etc.) can be applied to measure the strain in the sensor. Direct pressure sensing transducers can also be used which employ piezoelectric or piezoresistive materials.
Since all the sensors involve mechanical motion of the combined elastic and electrical sensor in response to the applied pressure, the dynamic response or transfer function will depend on size and stiffness of the sensor. A higher resonant frequency of the device can be obtained by making the the sensor as small as possible and using a stiff sensor material. The sensors will provide a linear output for frequencies up to 20% of the resonant frequency. Their useful bandwidth can be extended by standard signal processing techniques which can deconvolve the frequency dependent sensitivity of the sensor near resonance. Pressure sensors based on small bonded strain gauges appear to have the highest frequency response which extends to about 0.100 KHz (based on a natural resonance of 500 KHz). Piezoelectric sensors extend to several 10's of KHz. Both types can measure up to around 100,000 psi. The low frequency limit usually depends on the design of the elastic sensor. The electrical transducers will generally respond to static pressures.
In summary, pressure sensors are commercially available which can provide measurements to 100,000 psi with a bandwidth of 100 KHz. It is not likely that commercially available sensors can achieve significantly higher bandwidth and respond to the high pressures of interest here.
The test of providing a high pressure transducer which has a sufficient pressure range to measure a pressure consonant with a nuclear detonation is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,654,528 issued to Cloud, Jr. et al; PA1 U.S. Pat. No. 3,859,519 issued to Weischedel; PA1 U.S. Pat. No. 3,831,028 issued to Kerlman et al; PA1 U.S. Pat. No. 4,044,258 issued to Frangel; PA1 U.S. Pat. No. 4,577,510 issued to Bur et al; PA1 U.S. Pat. No. 4,366,714 issued to Adorni; PA1 U.S. Pat. No. 3,970,862 issued to Edelman et al; PA1 U.S. Pat. No. 3,894,243 issued to Edelman et al; and PA1 U.S. Pat. No. 3,940,974 issued to Taylor.
The above-cited patents disclose state-of-the-art nuclear explosion detectors and pressure sensor systems. To measure the extreme changes in pressure that characterize a nuclear detonation, one must have a pressure transducer with an extremely wide pressure range that accommodates changing conditions. Mechanical sensors possess inherent limitations that prevent them from having adequate pressure ranges for this use. Hybrid optical-mechanical pressure sensors also suffer from limited bandwidth. Purely optical pressure sensors in which some optical material property is modified by pressure have the potential for large bandwidth and greater pressure range. In addition, they offer immunity from EMI which can be a problem. Such sensors have been used extensively for static pressure measurement but have not been applied to the measurement of dynamic pressures such as those encountered in shock waves.
In view of the foregoing discussion, it is apparent that there currently exists the need for an optical high pressure sensor system which has a sufficient pressure range to make the requirement measurements on silo models. The present invention is intended to satisfy that need.